1. Field of the Invention The invention relates to an anti-static grounding device for use by persons assembling, maintaining, repairing or operating electronic equipment or the components thereof. In a preferred embodiment, the device comprises an anti-static wrist strap with attached wrist watch.
2. Description of the Prior Art
The sensitivity to static electricity of electronic equipment and its components, such as printed circuit boards, has increased considerably due to the decrease in the level of voltages required to operate printed circuit boards, which in some instances is in the amount of one volt. Printed circuit boards are expensive, and typically can cost in the range of $500.00 to $20,000.00 or more. In the manufacturing of such devices it is possible, through grounding mats and other precautions, to control the environment so as to reduce the exposure to static electricity and possible damage to the boards to an acceptable level. Such printed circuit boards and other sensitive equipment, after assembly, are typically packaged in an enclosure of anti-static material. When it is desired to replace a printed circuit board or other electronic component in the field, the repairman has to remove the component from its protective wrapping for installation into the equipment. It is difficult to control static electricity in the field working environment, and the charge carried or generated by the person installing the electronic components.
Various techniques have been utilized to attempt to alleviate damage to sensitive equipment or components due to the presence of static electricity. One method utilized in the prior art is to provide a grounded, conductive mat on the work surface, or on the floor. Another example of such a device is a touch bar device attached to the equipment which the operator touches before operating the equipment. A device of this type is disclosed in my co-pending application Ser. No. 735,870, now U.S. Pat. No. 4,586,106, issued Apr. 29, 1986.
While these devices are useful in controlled areas, it is often impractical to use them, and environmental conditions can exist that render them ineffective when handling sensitive electronic components.
It is also common to use a conductive wrist band to effectuate grounding of accumulated static electricity from a person assembling, maintaining, repairing or operating electronic equipment. The wrist strap is commonly worn by the person involved with the equipment, and is usually put on prior to such involvement.
It has been recognized that a conductive wrist band attached to ground can present a hazard due to an electrical short occurring in the ground circuit, thereby presenting a shock or electrocution hazard to the wearer, and various devices have been developed with safety features to attempt to alleviate these dangers.
Conductive wrist band devices are normally connected to ground by way of a grounding cord. As a safety feature, an inline resistor may be included in the cord or in the device itself to protect the wearer from high voltage conditions, which may exist when the grounding cord is disconnected from the device (and the conductive element is exposed to possible high voltage electrical circuits) or when the cord itself is accidentally connected to a high voltage source, or an electrical fault exists in the circuit to which the ground is connected.
Prior art wrist devices are typically constructed of a conductive material that is placed into electrical contact with the skin of the person wearing the device. The conductive material may take the form of a metallic plate on the inside of a wrist band, and may be a carbon-loaded conductive plastic or carbon-loaded conductive fabric. The prior art device of one popular wrist strap is of open mesh elastic woven fabric, with an electrically conductive plate in contact with the wearers skin, attached to a layer of fabric, with conductive metallic threads, a rivet in contact with the plate passing through the fabric, with an insulative plate on the outside of the strap, and with an outer exposed stud engaged with the rivet. Such devices rely on the wrist strap having an outer surface that is insulated from the conductive inner surface.
These prior art devices that rely on an electrically conductive material in contact with the skin, usually specify a conductive material having a surface resistivity below 100,000 ohms per square.
The Vandermark U.S. Pat. No. 4,459,663 describes a typical wrist device for draining off static electricity, which comprises an electrically conductive inner surface and an electrically insulated outer layer. Electrical contact to the skin of the user is made by a conductive metallic plate on the inside surface of the device. The electricity is then passed through an inline resistor, packaged within a casing, and to an exposed conductive eyelet portion in a dielectric cover on the case. The conductive eyelet portion may be attached to ground by the connection of a suitable grounding cord. This device is not entirely satisfactory, in that the inline resistor, of conventional form, which is included inside the casing, is costly, leads to an increase in the bulk and size of the device and is readily subject to contamination. It is also undesirable, as described in the Vandermark patent, to provide a case made of two dissimilar materials to encapsulate a conventional resistor. This component case can allow contaminating substances (water, perspiration, etc.) to enter the case, shorting out the resistor and destroying the usefulness of the resistor and thereby exposing the user to possible electrical shock.
In addition, metal conductive plates are uncomfortable and because of their rigidity, can separate from time to time from the skin resulting in intermittent static grounding. This type of device is often not worn by the user because of its bulk, and the potential safety hazards that can develop.
The U.S. Patent to Christiansen, et al. U.S. Pat. No. 4,398,227 describes a conductive elastomeric fabric and body strap. The device is formed of an insulative and elastomeric yarn, which is formed into interlocking loops on both the inner and outer surfaces. The device is applied by slipping it over the wrist, and due to its elastomeric properties, it conforms to the shape of the wrist of the wearer. The device, unfortunately, lacks the necessary safety features to protect the wearer from shock in the case of electrical fault, which could result when the yarn becomes wet or soaked with perspiration, which would cause its insulative qualities to disappear, and a potentially dangerous condition could result.
For a static grounding wrist band to function optimally, it should have the following characteristics:
It should be attractive and easily worn to induce the user to wear it at all times when working on static sensitive components.
It should make proper electrical connection between the wrist and the ground cord, particularly at the point of interface between the skin and the inside of the strap. The better this electrical interface, the better and more efficacious the grounding strap will be.
Total resistance between the wrist and the grounding portion of the wrist strap should not exceed (typically) 10 megohms as measured with a bias voltage of at least 10 volts. This resistance of 10 megohms or less will allow static generated charges (either generated through triboelectric forces or through changes in the wearer's capacitance [transient voltage]) to drain to ground in less than 0.04 seconds, a typically safe drain time to prevent the accumulation of static charges on the wearer.
The wrist strap should present no direct electrical contact with the skin either when the ground cord is attached or detached to prevent accidental shock or electrocution to the wearer if the conductive portion were to come in contact with high electrical voltages. Ideally, an in-line resistance of at least 0.25 megohms or higher should be included to assure protection against accidental shock.
During use of any of the prior art wrist straps, the user is instructed to first remove any wrist watch that he or she may be wearing, as the watch may have an unsafe conductive band or case. This is an inconvenience to persons who need to know the time and/or prefer to wear a watch at all times, and often results in those persons failing to utilize a static-draining wrist strap, and causing resultant damage to the electronic equipment. The prior art does not disclose, and it has not heretofore been proposed, to provide a static dissipative grounding strap, which may be attached to a wrist watch which has and must have, for safety reasons, a protective resistive element between the skin and an exposed conductive snap when worn under diverse working and living conditions, and when the ground cord is unconnected. The prior art also does not provide for an adapter kit for converting a conventional wrist watch into a static grounding device for use by persons who wish to wear their own personal wrist watch while involved with electrically sensitive equipment.